To obtain an image of a sample by electron microscopy, the sample is scanned with an electron beam and the electrons reflected or emitted from the sample are detected. The sample is usually scanned within a vacuum to maintain the vacuum integrity of the electron optics column and to permit the operation of electron detectors.
Scanning samples with an electron beam within a vacuum presents many problems. Biological specimens cannot survive in a vaccum for the length of time needed to obtain an accurate image. Wet specimens experience evaporation of their fluid content in the vacuum before an accurate image can be obtained. Objects that outgas at high vacuum require special consideration. Nonconducting samples accumulate a surface charge which obscures the details of the sample's surface and lowers the resolution of the image obtained. To avoid the problem of accumulating charge samples can be coated with a thin layer of metal to enable them to conduct away the charge. The metal coating however, can damage samples such as semiconductors and therefore often eliminates the possibility of nondestructive examination.
In an early work on environmental microscopy (W. C. Lane, "The Environmental Control Stage", Scanning Electron Microscopy/1970, pp. 43-48) Lane used an Everhart-Thornley system (a biased scintillator/photomultiplier) to detect electrons emitted from a specimen in a gaseous environment in a scanning electron microscope. He claimed that the secondary electrons emitted from the specimen surface ionized the cover gas (water vapor) producing free electrons that were then collected by the Everhart-Thornley detector. He concluded that, through this mechanism, the gas was responsible for amplification of the secondary electron signal prior to scintillation events in the Everhart-Thornley detector.
Lane's claims and conclusions are invalid, because useful amplification of the secondary electron signal was impossible in his experimental setup. The ionization energies (thresholds) of both constituents of water, hydrogen and oxygen, are too high to be ionized by the emitted secondary electrons and the claimed amplification process can only occur in the area immediately above the specimen where there is substantial gas pressure. The bias field of the Everhart-Thornley system cannot raise the energy of the secondary electrons sufficiently to allow for secondary electron signal amplification. The secondary electrons must acquire enough energy to ionize the gas while still in the gas filled region and the metal collar around the specimen reduces the strength of the bias field in this region. The gas filled region also only takes up a very small fraction of the vertical distance between the detector and the specimen and the secondary electrons can therefore only acquire a very small fraction of the total maximum potential difference between the detector and specimen before leaving the gas filled region. Thus the electron energies could not be raised to a high enough level to ionize the gas inside the collar.
The most likely explanation of Lane's observations of gas induced signal amplification is that higher energy emitted backscattered electrons ionized the gas in the collar. The resulting ionization products (electrons and/or ions) were detected by the scintillator.
Danilatos, Micron and Microscopa Acta 14(4):307-318 (1983) discloses that gas can act as a detection medium (citing the above Lane reference) using a conventional detector, and that it is possible to collect charge carriers with a low bias, or even no bias at all, on the detector. Danilatos says,
"The scope and efficiency of the gaseous detector device depends on a number of variables such as: (a) nature of the gas, (b) pressure of the gas, (c) temperature of the gas, (d) electrode configuration, (e) electrode bias, (f) accelerating voltage, (g) intensity of the primary beam current, (h) scanning speed, (i) nature of specimen." PA1 ". . . it was not possible to obtain a secondary electron image for a direct comparison. Alternatively some sort of trajectory contrast might be present. Furthermore, no external bias was applied, and it is not known to what extent the specimen and the attached electrode might have been self-biased. The results presented indicate that additional information can be obtained in the new system but the precise nature of this information requires further investigation."
Danilatos concludes that a detailed study is needed to understand various ion pair phenomena in the system.